1. Field of the Invention
The present invention relates to a radio communication system, and more particularly to a method of re-send control and transmission control of data.
2. Description of the Related Art
In the personal radio communication apparatus as represented by mobile telephone, mobility of the apparatus is demanded. Therefore it is required to be capable of communicating while moving, and to be portable easily, that is, the apparatus itself should be reduced in size.
In the latter point, in particular, it is important to reduce the size of the battery which occupies a considerable portion of the weight and volume of the terminal, and, in turn, it is important to reduce the power consumption of the apparatus.
On the other hand, in the conventional radio data transmission system, generally, the reception side checks if there is any error in data or not when receiving, and requests re-send to the transmission side if error is found in the data, and the transmission side re-sends according to this request.
Herein, the re-send control method in the conventional radio data transmission system is described while referring to FIG. 1.
FIG. 1 shows a conventional radio data transmission system. In FIG. 1, a base station 101 which is usually wired to the network transmits data by radio to a second mobile station 102. A service area 103 shows a range in which the base station 101 can transmit data.
In such radio data transmission system, hitherto, data is divided into several units (this unit is called a packet), and a parity bit for error detection is added to each packet, and is transmitted from the base station 101 to the service area 103 by radio, and the mobile station 102 receives it. In the mobile station 102, every time a packet (data packet) is received, it is checked if error is contained in the received packet or not, and if an error is found, it requests re-send of the packet containing the error immediately to the base station 101, and the base station 101 re-sends this packet containing error to the mobile station 102 according to the re-send request.
The sequence of re-send control method is explained below by referring to an example of "stop-and-wait" method shown in FIG. 2.
A sequence number and redundancy bits for detecting error are added to a data packet. First, N-th data packet is transmitted from the base station 101 to the mobile station 102. In the mobile station 102, presence of error is checked by using the redundancy bits for detecting and correcting error. When no error is found, as shown in FIG. 2, ACK (affirmative response) showing that the N-th data packet is received correctly is transmitted to the base station 101.
The base station 101, when receiving ACK, transmits the next (N+1)-th data packet to the mobile station 102. In the second base station 102, checking error similarly, and if error is found, as shown in FIG. 2, NAK (negative response) showing that error is contained in the (N+1)-th data packet is transmitted to the first base station 101.
The base station 101, when receiving NAK, re-sends the (N+1)-th data packet to the mobile station 102. In the second base station 102, checking error similarly, and when no error is found, as shown in FIG. 2, ACK showing that the (N+1)-th data packet is received correctly is transmitted to the base station 101. After receiving ACK, the base station 101 transmits next data to the mobile station 102.
Such operation is repeated to transmit data.
As the method of re-send control, aside from such "stop-and-wait" method, "go-back-N" method and selective repeat method are known among others, but basically the control method is same in that data is retransmitted when NAK is received.
Incidentally, in data transmission, such data re-send means that the mobile station 102 cannot leave the service area 103 of the base station 103 until the data re-send is over. This is very inconvenient, if the mobile station 102 is a mobile terminal, because the moving is limited.
To prevent such problem, a method of changing over the base station to transmit data is known. As a representative example, the system of portable telephone is known, which is briefly described by reference to FIG. 3.
In FIG. 3, the mobile station 101 which is a mobile terminal (a portable telephone in this case) is supposed to be first communicating in the service area 103 of the base station 101 which is a certain regional base station. Suppose this mobile station 102 is moved to a service area 105 of another base station 104 which is a different regional base station in the midst of communication. At this time, the network 106 of telephone system has a function of detecting that the mobile station 102 is moved from the service area 103 of the base station 101 to the service area 105 of the other base station 104, and making the communication to the mobile station 102 available through the service area 105 of the other base station 104, thereby changing over the station to communicate with the mobile station 102 from the base station 101 to the other station 104. It is hence possible to satisfy the requirement for mobility of the system in the personal communication apparatus.
In the radio communication system for transmitting data by packet communication or the like, between the data (data packet) sending side and receiving side, data transmission, data error check, re-send request if error is found, and re-send of data requested to send again are carried out in a series of (one) communication occasion. This is extremely useful when real time is required in communication, but real time is not required in all data communications. For example, in distribution of less urgent message, or newspaper articles or magazine articles to subscribers, it is not so urgent that the data should be sent again immediately if error is found.
Seeing that there are many applications in which real time is not required, data re-send is not always required to be done instantly. In the conventional system, it is done automatically and instantly. For immediate re-send control, the demodulator in the personal communication apparatus of the terminal possessed by the user must continue to be operated at high speed after the data is sent. As mentioned above, in personal communication, reduction of power consumption is important. However, even when real time is not required, the circuit for error checking is operated, or the re-send requesting circuit is operated, and the conventional system is disadvantageous from the viewpoint of power consumption.
In addition, from the viewpoint of effective use of radio frequency band that must be shared by multiple users (effective use of radio resources), although it is particularly needed in personal communication, re-send control is done immediately even when it is not necessary that data should be sent without error in real time, and the channel is occupied without need, and the effective use of radio resources is impaired, and the use by others is blocked.
Recently, in particular, the multimedia is attracting attention, and the radio communication system capable of exchanging moving picture, voice and data is increasing its importance, and a colossal quantity of data must be sent in a short time, and it requires transmission speed of as high as 100 Mbps or more.
For such high speed transmission, a frequency band of 100 MHz is needed, and when using such broad frequency band as transmission path for radio communication, as a matter of course, re-send control for such less urgent data is a mere waste of radio resources. Its improvement is keenly demanded.
Further, in the case of radio transmission, effective utilization of channel by rational transmission of data is also a natural request in the society. In the conventional radio communication, the mainstream is the system using one up-link channel and one down-link channel having same transmission speed, and data is transmitted by using a pair of channels.
On the other hand, the data to be transmitted is coded in order to compress the volume as much as possible in the case of large capacity data such as picture. There is provided an example of data compression in which original data is first sent and then only changed portions of the data are sent. According to this example, the data is classified into initial data and additional data, and then the initial data and the additional data are respectively coded. For example, in image coding of video picture, about 60 frames are needed per second, and according to this method, the first frame is transmitted as it is, and from the subsequent frames, only the difference from the previous frame is sent. This method is utilized in MPEG2 (Moving Picture Experts Group 2) and others. In another method, incidentally, a specific range on the screen is regarded as one object, and the moving direction of this object in the next frame is transmitted. In coding of voice, having a common dictionary (code book) at the transmission side and reception side, it is proposed to decrease the quantity of data to be transmitted by consulting the dictionary (CELP: Code Excited Linear Prediction method). When coding the voice, its spectrum is analyzed, and the optimum patterns is referred to from the code book depending on the result of analysis, and transmitted. Herein, the common dictionary stores fixed initial data that is not changed during communication, and only the result by referring to the code book is the additional data that is changed during communication, and only this additional data is usually transmitted in communication.
More recently, Telescript is known as one of the representative advanced forms of language for communication. It is intended to transmit simultaneously the message of the data to be transmitted, and the program describing the processing about the content of the message or the message itself. The message is the initial data, and by processing this message as initial data by the program, the data can be taken out. The message is the initial data, and the program is the additional data.
Thus, in data transmission, the forms of initial data and additional data are used in many of the coding and describing methods. As the method of transmission data in such forms by radio communication, hitherto, the method shown in FIG. 4 is employed.
As shown in the diagram, the data generated in a data source 81 is divided into initial data and additional data, and coded in an encoder 82. The coded initial data and additional data are multiplexed in time according to a predetermine rule in a multiplexer 83. At this time, whether the transmitted data is initial data or additional data is distinguished by the data bit added thereto. The multiplexed signal is modulated into a radio signal in a transmitter 84, and is transmitted from an antenna 85.
At the receiving side, the radio signal received by an antenna 86 is demodulated in a receiver 87, and is delivered from an output terminal 89. In a decoder 88, the data is decoded by distinguishing whether the demodulation signal series is initial data or additional data by the added data bit.
Thus, hitherto, data is transmitted in one channel regardless of the type of data.
Other prior art is explained by referring to FIG. 5.
In mobile radio communication, a method of disposing the service area (cell) covered by one base station hierarchically is known. In FIG. 5, a base station 201 covers a wide area 202. A base station 203 covers a small area 204, and a base station 205 covers a small area 206.
The base stations 203 and 205 cover the small areas 204 and 206 limited by spot beam, in which the transmission distance is short, and a large capacity of data can be transmitted at high speed. The base station 201 covers by a wide-range beam, there are many users in the wide area 202, and the band that can be transmitted by one user is limited, and data is transmitted only at low speed.
In such hierarchical cell arrangement, conventionally, images of large capacity are communicated by spot beam, while voice of small data quantity, or rough image or still picture of small number of pixels is transmitted by wide-range beam, and thus the roles are generally divided. In other manner of using the hierarchical cells, for example, only exchange of radio control signal necessary for radio communication is done by wide-range beam, and data communication is done by spot beam only.
In the prior art shown in FIG. 4, regardless of the content of the data, data is transmitted by using the channels of specific transmission speed. Generally, the initial data is large in the data quantity to be transmitted at a time as compared with the additional data. Thinking of image, as the initial data, the whole data of one frame must be directly transmitted, while the additional data is smaller in the data quantity as compared the initial data. To transmit moving picture without time delay, a high speed channel enough to transmit the initial data without delay is required, but such high speed channel is not required for transmitting additional data, and the frequency cannot be utilized effectively. From the viewpoint of effective utilization of frequency, if the circuit of such a transmission speed as to transmit additional data only is used, it takes a very long time to transmit the initial data, and therefore the data cannot be transmitted in real time, and time delay is caused.
A method of varying the transmission speed by one channel is also proposed, but anyway since the same channel is used physically, it requires an apparatus for realizing maximum transmission speed in a service area. To realize variable transmission speed, different transmission speeds must be realized by using a same apparatus in a same frequency band. It does not matter much as far as the radio of data quantity to be transmitted per unit time is small between the initial data and additional data, but it is a serious problem if the data differs by two digits or more. Supposing the transmission speed of initial data to be 10 Mbps and that of additional data to be 10 Kbps, the frequency band for transmitting additional data is sufficient at around 1 GHz. By contrast, however, since the initial data is high in transmission speed, it is difficult to transmit in 1 GHz band. This is because that the frequency bandwidth assigned for such data communication service is, at most, about several MHz to several tens of MHz around 1 GHz, considering the present situation of utilization of radio waves. To transmit the initial data in such frequency band, one user occupies 10 MHz of this bandwidth, which is nearly impossible to realize. Considering transmission of initial data, it may be possible to use a millimeter wave band of about 60 GHz which is a presently free frequency band. In the millimeter wave band, the area that can be covered by one base station is about several to several tens of meters due to the wave propagation characteristic. To transmit at variable transmission speeds in this frequency band, multiple base stations must be installed, which lead to increase of cost for infrastructure.
On the other hand, in the prior art in FIG. 5, depending on the type of data such as image and voice, it is determined whether to transmit by spot beam or to transmit by wide range beam. Or the spot beam or wide range beam is determined depending on whether it is control data on the radio communication protocol or data to be transmitted by the user. In this case, the problem is that the available service is extremely limited in the point of wide range beam.